Gore and frightening images could certainly elevate the entire "darkness" of the game. The way it sits now, I've never really felt the threat from the enemies, because I've never seen it. Imagine if the first introduction to the fallen was witnessing a defenseless person from Earth being torn limb from limb by a Archon Priest. That's a lot darker and more impactful than a quick cutscene of him looking at you and running away. The closest thing this game has come to with a dark theme is the Hive and its lore, which is PG at most.